Laterally-diffused metal-oxide semiconductor transistor and method therefor

ABSTRACT

A transistor includes a trench formed in a semiconductor substrate with the trench having a first sidewall and a second sidewall. A gate region includes a conductive material filled in the trench. A drift region having a first conductivity type is formed in the semiconductor substrate adjacent to the second sidewall. A drain region is formed in the drift region and separated from the second sidewall by a first distance. A dielectric layer is formed at the top surface of the semiconductor substrate covering the gate region and the drift region between the second sidewall and the drain region. A field plate is formed over the dielectric layer and isolated from the conductive material and the drift region by way of the dielectric layer.

BACKGROUND Field

This disclosure relates generally to semiconductor devices, and morespecifically, to laterally-diffused metal-oxide semiconductor (LDMOS)transistors and method of forming the same.

Related Art

Traditional semiconductor devices and semiconductor device fabricationprocesses are evolving. For example, metal oxide semiconductorfield-effect transistors (MOSFETs) are used in a variety of differentapplications and electronic products—from sewing machines to washingmachines, from automobiles to cellular telephones, and so on. As processtechnology progresses, these semiconductor devices are expected toreduce in size and cost while increasing performance. However,challenges exist in balancing size, cost, and performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIG. 1 through FIG. 7 illustrate, in simplified cross-sectional views,various stages of manufacture of an example laterally-diffusedmetal-oxide semiconductor (LDMOS) transistor in accordance with anembodiment.

FIG. 8 and FIG. 9 illustrate, in simplified plan views, the exampleLDMOS transistor at a stage of manufacture in accordance with anembodiment.

DETAILED DESCRIPTION

In recent years, automotive, industrial, and consumer applications havehad increasing demands on smart power technologies which integratedigital, analog, and high-voltage power transistors on a single chipaimed to lower manufacturing costs. In semiconductor manufacturing,silicon area is at a premium. Desirable improved on-resistance*area(RonA) values of power transistors with reduced silicon area impact isone example of technological improvement occurring in a trench-basedtransistor as described herein.

Generally, there is provided, a trench-based laterally-diffusedmetal-oxide semiconductor (LDMOS) transistor. A trench formed in asemiconductor substrate is filled with a conductive material to form agate region. A gate dielectric is formed as a liner in the trenchisolating the gate region from the substrate. A source region is formedat the surface of the semiconductor substrate adjacent to the trench. Adrain region is formed at the surface of the semiconductor substrateseparated from the trench by a predetermined distance. A dielectriclayer is formed over the gate region and spans the predetermineddistance over a drift region between the trench and the drain region. Afield plate is formed over a portion of the dielectric layer. The fieldplate and dielectric layer have dimensions chosen to achieve higherbreakdown voltages and improved RonA.

FIG. 1 through FIG. 7 illustrate, in simplified cross-sectional views,various stages of manufacture of an example laterally-diffusedmetal-oxide semiconductor (LDMOS) transistor 100 formed in accordancewith an embodiment.

FIG. 1 illustrates in a simplified cross-sectional view, example LDMOStransistor 100 at a stage of manufacture in accordance with anembodiment. At this stage, transistor 100 includes a silicon-basedsubstrate 102, a patterned hard mask 116 formed over the substrate 102,a trench 104 formed in the substrate 102, and a gate dielectric 106formed at the sidewalls and bottom of the trench 104. In thisembodiment, the substrate 102 is formed as a P-type doped siliconsubstrate having a P-type conductivity type. Substrate 102 may be formedas a P-type doped silicon substrate having an N-type epitaxial layer.Substrate 102 may alternatively be formed from other suitablesilicon-based substrates such as gallium arsenide, silicon germanium,silicon-on-insulator (SOI), silicon, monocrystalline silicon, the like,and combinations thereof, for example.

In this embodiment, an oxide layer 112 is formed on the on substrate102, and a nitride layer 114 is formed on the oxide layer 112. Thenitride/oxide layers together are patterned to form hard mask 116. Inother embodiments, other suitable materials and combinations thereof maybe used to form hard mask 116. In this embodiment, the trench 104includes a first sidewall 108, a second sidewall 110, and a bottomsurface. Etch techniques such as reactive ion etching (RIE) may beemployed to form trench 104, for example. In this embodiment, the gatedielectric 106 is formed on exposed surfaces of trench 104 essentiallyforming a liner layer at sidewalls 108-110 and bottom surfaces of trench104. The gate dielectric 106 may be formed from any suitable gatedielectric material (e.g., silicon dioxide). The gate dielectric 106 maybe formed as a grown layer, deposited layer, or combination thereof.

FIG. 2 illustrates in a simplified cross-sectional view, example LDMOStransistor 100 at a subsequent stage of manufacture in accordance withan embodiment. At this stage, a conductive material is deposited tosubstantially fill trench 104 and to form a gate region 202. Aplanarization operation is performed to level the conductive materialwith 114 layer. In this embodiment, the planarization operation mayinclude a chemical-mechanical planarization (CMP) process. In otherembodiments, the planarization operation may include a wet etch process.A subsequent etch operation may be performed to substantially level theconductive material with the top surface of the substrate to form thegate region 202. The gate region 202 may be formed from a suitableconductive material such as a polysilicon material or a metal material,for example. In this embodiment, the gate region 202 is configured andarranged to serve as a gate electrode for the example LDMOS transistor100. In other embodiments, the gate region 202 may be formed from otherconductive materials.

FIG. 3 illustrates in a simplified cross-sectional view, example LDMOStransistor 100 at a subsequent stage of manufacture in accordance withan embodiment. At this stage, a planar top surface 306 of substrate 102is formed, and body region 302 and drift region 304 are formed.

In the embodiment depicted in FIG. 3, the hard mask 116 at the topsurface of the substrate 102 is removed forming the planar top surface306. In this embodiment, the gate region 202 is substantially planarwith the planar top surface 306. In other embodiments, the gate region202 may be slightly recessed or slightly protruding from the planar topsurface 306. The gate dielectric 106 remaining in the trench serves toisolate the gate region 202 from the substrate 102.

In this embodiment, body region 302 is formed as a P-type well dopantimplanted region in substrate 102, adjacent to sidewall 108. The bodyregion 302 may be characterized as a P− (minus) body region. Driftregion 304 is formed as an N-type well dopant implanted region insubstrate 102, adjacent to sidewall 110. The drift region 304 may becharacterized as a N− (minus) drift region. In some embodiments, thedrift region 304 may be formed from an N-type epitaxial layer ofsubstrate 102.

FIG. 4 illustrates in a simplified cross-sectional view, example LDMOStransistor 100 at a subsequent stage of manufacture in accordance withan embodiment. At this stage, a dielectric layer 402 is formed at thetop surface of substrate 102. Dielectric layer 402 formed at the topsurface of the substrate 102 is deposited and patterned to cover thegate region 202 and portions of the body and drift regions 302 and 304.In this embodiment, dielectric layer 402 may be formed from a dielectricmaterial (e.g., silicon nitride) suitable for serving as a self-alignedmask when forming subsequent process structures (e.g., source/drainregions, salicide layer). Dimensions (e.g., thickness, width, length) ofthe dielectric layer 402 may be chosen for optimal reduced surface field(RESURF).

FIG. 5 illustrates in a simplified cross-sectional view, example LDMOStransistor 100 at a subsequent stage of manufacture in accordance withan embodiment. At this stage, source and drain regions 502 and 504 areformed, and body tie region 506 is formed. After body region 302 anddrift region 304 are formed, N-type dopants are implanted to form sourceand drain regions 502 and 504 respectively. Source and drain regions 502and 504 may be characterized as N+ (plus) source/drain regions,respectively. Source region 502 is formed adjacent to sidewall 108 andP-type dopant is implanted to form body tie region 506 allowingelectrical connectivity with body region 302. Body tie region 506 may becharacterized as a P+ (plus) body tie region. In this embodiment, bodytie region 506 abuts source region 502. Drain region 504 is formedseparate from sidewall 110 by a lateral distance 508. As the lateraldistance 508 increases, corresponding breakdown voltages increase. Inthis embodiment, the lateral distance 508 between the sidewall 110 anddrain region 504 may be in range of 0.5 microns to 10.0 microns, forexample. In some embodiments, the lateral distance 508 may be less than0.5 microns or greater than 10.0 microns.

FIG. 6 illustrates in a simplified cross-sectional view, example LDMOStransistor 100 at a subsequent stage of manufacture in accordance withan embodiment. At this stage, salicide regions 602 are formed at a topsurface of source and drain regions 502 and 504, and body tie region506. A metal thin film layer (e.g., titanium, platinum, tungsten) isdeposited and reacted with the exposed top surface of the source anddrain regions 502 and 504, and body tie region 506 to form the salicideregions 602. The salicide regions serve to form a high conductivitycontact region at the top surface of source and drain regions 502 and504 and body tie region 506. In this embodiment, the term salicide asused herein may also refer to self-aligned silicide.

FIG. 7 illustrates in a simplified cross-sectional view, example LDMOStransistor 100 at a subsequent stage of manufacture in accordance withan embodiment. At this stage, the example LDMOS transistor 100 includescontacts 702-708, electrode terminals 710-714, and horizontal fieldplate 716. In this embodiment, horizontal field plate 716 and contact708 together form a pseudo L-shaped field plate as depicted in FIG. 7.

In this embodiment, an inter-level dielectric (ILD) region 720 is formedover the dielectric layer 402 and the salicide regions 602. The ILDregion 720 may be formed from a series of deposited oxide layers such astetraethyl orthosilicate (TEOS). For example, a first oxide layer of ILDregion 720 may be deposited, patterned, and etched to expose portions ofgate region 202, salicide regions 602, and dielectric layer 402. Afterthe first oxide layer is patterned and etched, contacts 702-708 areformed. Contacts 702-708 may be formed from any suitable conductivematerial such as copper, gold, silver, aluminum, nickel, tungsten, andalloys thereof, for example. Contacts 702-706 provide a conductiveconnection to the source and body tie regions 502 and 506, gate region202, and drain region 504 respectively. Contact 708 is formed such thata bottom surface of the contact 708 abuts a top surface of thedielectric layer 402. In this embodiment, contact 708 serves as avertical field plate portion having a first edge 718 overlapping aportion of the gate region 202 at sidewall 110.

After the contacts 702-708 are formed, a conductive layer is deposited,patterned, and etched to form electrode terminals 710-714, andhorizontal field plate 716. The electrode terminals 710-714 and thehorizontal field plate 716 may be formed from any suitable conductivematerial such as copper, gold, silver, aluminum, nickel, tungsten, andalloys thereof, for example. In this embodiment, source electrodeterminal 710 is connected to source and body tie regions 502 and 506 byway of contact 702 and salicide region 602, gate electrode terminal 712is connected to the gate regions 202 by way of contact 704, and drainelectrode terminal 714 is connected to drain region 504 by way ofcontact 706 and salicide region 602.

Horizontal field plate 716 is directly connected to contact 708 formingthe pseudo L-shaped field plate. A portion of horizontal field plate 716overlaps the underlying drift region 304 between the gate region 202 anddrain region 504 by an overlap distance 726. The overlap distance 726may be chosen for optimal RESURF. In this embodiment, the overlapdistance 726 may be in a range of 40% to 60% of the lateral distance508, for example. In some embodiments, the overlap distance 726 may beless than 40% or greater than 60% of the lateral distance 508. In thisembodiment, the field plate is electrically isolated from the substrateby way of the dielectric layer 402. In some embodiments, the field platemay be connected to the source electrode terminal 710 or the gateelectrode terminal 712 by way of a metal interconnect layer, forexample.

In the embodiment depicted in FIG. 7, the electrode terminals 710-714and the horizontal field plate 716 are formed from a same conductivelayer. In other embodiments, the electrode terminals 710-714 and thehorizontal field plate 716 may be formed from different conductivelayers. After the electrode terminals 710-714 and the horizontal fieldplate 716 are formed, a subsequent oxide layer (e.g., TEOS) of ILDregion 720 may be deposited to cover exposed surfaces of the electrodeterminals 710-714, the horizontal field plate 716, and first oxidelayer.

FIG. 8 illustrates, in a simplified plan view, the example LDMOStransistor 100 at a stage of manufacture in accordance with anembodiment. In the embodiment depicted in FIG. 8, the LDMOS transistor100 is formed as an oval shaped transistor with source region 802, drainregion 804, gate region 806, and dielectric layer 808 corresponding,respectively, to the source region 502, drain region 504, gate region202, and dielectric layer 402 of FIG. 7. The dielectric layer 808 isshown as a transparent region allowing underlying details to be visible.In this embodiment, source region 802 is surrounded by gate region 806.Drain region 804 is formed separate from the gate region 806 andsurrounds the gate region 806 at the lateral distance 508 as depicted inFIG. 7. Dielectric layer 808 is formed covering cover the gate region806, portions of the source and drain regions 802 and 804, and the driftregion (not shown) between the gate region 806 and the drain region 804.The dashed outlined areas 810 and 812 depict openings in the dielectriclayer 808. Example contacts 814 provide connections to source, drain,and gate regions 802-806, for example. Features such as the electrodeterminals and field plate are not shown.

FIG. 9 illustrates, in a simplified plan view, the example LDMOStransistor 100 at a stage of manufacture in accordance with anembodiment. In the embodiment depicted in FIG. 9, the LDMOS transistor100 of FIG. 8 is shown with dielectric layer 808 as an opaque (shaded)region preventing underlying details to be visible. In this embodiment,a source region portion 902 is visible through opening 812 of dielectriclayer 808 and gate region portions 904 are visible through openings 810of dielectric layer 808.

Generally, there is provided, a transistor including a trench formed ina semiconductor substrate, the trench having a first sidewall and asecond sidewall; a gate region comprising a conductive material filledin the trench; a drift region formed in the semiconductor substrateadjacent to the second sidewall, the drift region having a firstconductivity type; a drain region formed in the drift region, the drainregion separated from the second sidewall by a first distance; adielectric layer formed at the top surface of the semiconductorsubstrate covering the gate region and the drift region between thesecond sidewall and the drain region; and a field plate formed over thedielectric layer, the field plate isolated from the conductive materialand the drift region by way of the dielectric layer. A first edge of thefield plate may overlap a portion of the gate region and a second edgeof the field plate may extend a second distance over the drift region,the second distance less than the first distance. The field plate mayinclude a horizontal portion and a vertical portion, the horizontalportion separated from the dielectric layer by an interlayer dielectric(ILD), and the vertical portion contacted to the horizontal portionproximate to the first edge and extends from a bottom surface of thehorizontal portion to a top surface of the dielectric layer. Thetransistor may further include a gate dielectric disposed at sidewallsand bottom of the trench, the gate dielectric isolating the conductivematerial from the semiconductor substrate. The conductive material maybe formed from a polysilicon material and the field plate may be formedfrom a metal material. The transistor may further include a sourceregion formed in the semiconductor substrate adjacent to the firstsidewall, a portion of the dielectric layer overlapping the sourceregion. The transistor may further include a body region having a secondconductivity type formed in the semiconductor substrate adjacent to thefirst sidewall, the source region formed in the body region. Thetransistor may further include a salicide layer formed at the topsurface of the drain region and the source region, and wherein thedielectric layer is formed from nitride material and is configured toserve as a mask for the salicide layer. The transistor may furtherinclude a source terminal contacted to the source region, the sourceterminal connected to the field plate by way of a metal layer.

In another embodiment, there is provided, a method including etching atrench in a semiconductor substrate, the trench having a first sidewalland a second sidewall; filling the trench with a conductive material toform a gate region; forming a drift region in the semiconductorsubstrate adjacent to the second sidewall, the drift region having afirst conductivity type; forming a drain region in the drift region, thedrain region separated from the second sidewall by a first distance;patterning a dielectric layer at the top surface of the semiconductorsubstrate to cover the gate region and the drift region between thesecond sidewall and the drain region; and forming a field plate over thedielectric layer, the field plate isolated from the conductive materialand the drift region by way of the dielectric layer. A first edge of thefield plate may overlap a portion of the gate region and a second edgeof the field plate may extend a second distance over the drift region,the second distance less than the first distance. The conductivematerial may be formed from a polysilicon material and the field platemay be formed from a metal material. Forming the field plate may includeforming a vertical portion having a bottom surface contacting a topsurface of the dielectric layer and a top surface extending through aninterlayer dielectric (ILD); and forming a horizontal portion contactinga top surface of the vertical portion, the horizontal portion separatedfrom the dielectric layer by the ILD. The method may further includeforming a gate dielectric at sidewalls and bottom of the trench beforefilling the trench with the conductive material, the gate dielectricisolating the conductive material from the semiconductor substrate. Themethod may further include forming a source region in the semiconductorsubstrate adjacent to the first sidewall, a portion of the dielectriclayer overlaps the source region. The method may further include forminga body region having a second conductivity type in the semiconductorsubstrate adjacent to the first sidewall, the source region formed inthe body region. The method may further include forming a gate terminalcontacted to the gate region, and connecting the gate terminal to thefield plate by way of a metal layer.

In yet another embodiment, there is provided, a transistor including atrench formed in a semiconductor substrate, the trench having a firstsidewall and a second sidewall; a gate region including a conductivematerial filled in the trench; a gate dielectric disposed at sidewallsand bottom of the trench, the gate dielectric isolating the conductivematerial from the semiconductor substrate; a drift region formed in thesemiconductor substrate adjacent to the second sidewall, the driftregion having a first conductivity type; a drain region formed in thedrift region, the drain region separated from the second sidewall by afirst distance; a body region formed in the semiconductor substrateadjacent to the first sidewall, the body region having a secondconductivity type; a dielectric layer formed at the top surface of thesemiconductor substrate covering the gate region and the drift regionbetween the second sidewall and the drain region; and a field plateformed over the dielectric layer, the field plate isolated from theconductive material and the drift region by way of the dielectric layer.A first edge of the field plate may overlap a portion of the gate regionand a second edge of the field plate may extend a second distance overthe drift region, the second distance less than the first distance. Thetransistor may further include a source region having the firstconductivity type formed in the body region adjacent to the firstsidewall, a portion of the dielectric layer overlapping the sourceregion.

By now it should be appreciated that there has been provided atrench-based LDMOS transistor. A trench formed in a semiconductorsubstrate is filled with a conductive material to form a gate region. Agate dielectric is formed as a liner in the trench isolating the gateregion from the substrate. A source region is formed at the surface ofthe semiconductor substrate adjacent to the trench. A drain region isformed at the surface of the semiconductor substrate separated from thetrench by a predetermined distance. A dielectric layer is formed overthe gate region and spans the predetermined distance over a drift regionbetween the trench and the drain region. A field plate is formed over aportion of the dielectric layer. The field plate and dielectric layerhave dimensions chosen to achieve higher breakdown voltages and improvedRonA.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope of thepresent invention. Any benefits, advantages, or solutions to problemsthat are described herein with regard to specific embodiments are notintended to be construed as a critical, required, or essential featureor element of any or all the claims.

Furthermore, the terms “a” or “an,” as used herein, are defined as oneor more than one. Also, the use of introductory phrases such as “atleast one” and “one or more” in the claims should not be construed toimply that the introduction of another claim element by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim element to inventions containing only one such element,even when the same claim includes the introductory phrases “one or more”or “at least one” and indefinite articles such as “a” or “an.” The sameholds true for the use of definite articles.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

What is claimed is:
 1. A transistor comprising: a trench formed in asemiconductor substrate, the trench having a first sidewall and a secondsidewall; a source region formed in the semiconductor substrate adjacentto the first sidewall; a source terminal connected to the source region;a gate region comprising a conductive material filled in the trench,wherein the source region is surrounded by the gate region; a gateterminal connected to the conductive material; a drift region formed inthe semiconductor substrate adjacent to the second sidewall, the driftregion having a first conductivity type; a drain region formed in thedrift region, the drain region separated from the second sidewall by afirst distance; a drain terminal connected to the drain region; adielectric layer formed at a top surface of the semiconductor substratecovering the gate region and the drift region between the secondsidewall and the drain region; and a field plate formed over thedielectric layer, the field plate isolated from the conductive materialand the drift region by way of the dielectric layer and isolated fromthe source terminal, the gate terminal, and the drain terminal.
 2. Thetransistor of claim 1, wherein a first edge of the field plate overlapsa portion of the gate region and a second edge of the field plateextends a second distance over the drift region, the second distanceless than the first distance.
 3. The transistor of claim 2, wherein thefield plate includes a horizontal portion and a vertical portion, thehorizontal portion separated from the dielectric layer by an interlayerdielectric (ILD), and the vertical portion contacted to the horizontalportion proximate to the first edge and extends from a bottom surface ofthe horizontal portion to a top surface of the dielectric layer.
 4. Thetransistor of claim 1, further comprising a gate dielectric disposed atsidewalls and bottom of the trench, the gate dielectric isolating theconductive material from the semiconductor substrate.
 5. The transistorof claim 2, wherein the second distance is between 40% and 60% of thefirst distance.
 6. The transistor of claim 1, wherein a portion of thedielectric layer overlaps the source region.
 7. The transistor of claim6, further comprising a body region having a second conductivity typeformed in the semiconductor substrate adjacent to the first sidewall,the source region formed in the body region.
 8. The transistor of claim6, further comprising a salicide layer formed at the top surface of thedrain region and the source region, and wherein the dielectric layer isformed from nitride material and is configured to serve as a mask forthe salicide layer.
 9. A method comprising: etching a trench in asemiconductor substrate, the trench having a first sidewall and a secondsidewall; filling the trench with a conductive material to form a gateregion; forming a source region in the semiconductor substrate adjacentto the first sidewall; forming a source terminal connected to the sourceregion; forming a gate region comprising a conductive material filled inthe trench, wherein the source region is surrounded by the gate region;forming a gate terminal connected to the gate region; forming a driftregion in the semiconductor substrate adjacent to the second sidewall,the drift region having a first conductivity type; forming a drainregion in the drift region, the drain region separated from the secondsidewall by a first distance; forming a drain terminal connected to thedrain region; patterning a dielectric layer at a top surface of thesemiconductor substrate to cover the gate region and the drift regionbetween the second sidewall and the drain region; and forming a fieldplate over the dielectric layer, the field plate isolated from theconductive material and the drift region by way of the dielectric layerand isolated from the source terminal, the gate terminal, and the drainterminal.
 10. The method of claim 9, wherein a first edge of the fieldplate overlaps a portion of the gate region and a second edge of thefield plate extends a second distance over the drift region, the seconddistance less than the first distance.
 11. The method of claim 9,wherein the conductive material is formed from a polysilicon materialand the field plate is formed from a metal material.
 12. The method ofclaim 9, wherein forming the field plate comprises: forming a verticalportion having a bottom surface contacting a top surface of thedielectric layer and a top surface extending through an interlayerdielectric (ILD); and forming a horizontal portion contacting a topsurface of the vertical portion, the horizontal portion separated fromthe dielectric layer by the ILD.
 13. The method of claim 10, wherein thesecond distance is between 40% and 60% of the first distance.
 14. Themethod of claim 9, wherein a portion of the dielectric layer overlapsthe source region.
 15. The method of claim 14, further comprises forminga body region having a second conductivity type in the semiconductorsubstrate adjacent to the first sidewall, the source region formed inthe body region.
 16. A transistor comprising: a trench formed in asemiconductor substrate, the trench having a first sidewall and a secondsidewall; a source region formed in the semiconductor substrate adjacentto the first sidewall; a source terminal connected to the source region;a gate region comprising a conductive material filled in the trench,wherein the source region is surrounded by the gate region; a gatedielectric disposed at sidewalls and bottom of the trench, the gatedielectric isolating the conductive material from the semiconductorsubstrate; a gate terminal connected to the conductive material; a driftregion formed in the semiconductor substrate adjacent to the secondsidewall, the drift region having a first conductivity type; a drainregion formed in the drift region, the drain region separated from thesecond sidewall by a first distance; a drain terminal connected to thedrain region; a body region formed in the semiconductor substrateadjacent to the first sidewall, the body region having a secondconductivity type; a dielectric layer formed at a top surface of thesemiconductor substrate covering the gate region and the drift regionbetween the second sidewall and the drain region; and a field plateformed over the dielectric layer, the field plate isolated from theconductive material and the drift region by way of the dielectric layerand isolated from the source terminal, the gate terminal, and the drainterminal.
 17. The transistor of claim 16, wherein a first edge of thefield plate overlaps a portion of the gate region and a second edge ofthe field plate extends a second distance over the drift region, thesecond distance less than the first distance.
 18. The transistor ofclaim 17, wherein the second distance is between 40% and 60% of thefirst distance.